Concentrating seismic energy in a selected target point in an underground formation

ABSTRACT

Providing a set of seismic sources in a focusing array to enable concentrating seismic energy in a selected target point (k) in an underground formation, by performing a standard seismic survey of the underground formation using a set of impulsive seismic sources and a set of downhole seismic receivers j, and recording the signals u ij (t), selecting a position k to be the target point; obtaining from the signals u ij (t) the one-way travel times τ ik  from a number of sources i to the target point at the location k; and calculating t i =t 0 −τ ik  to obtain the activating times t i  at which the impulsive seismic sources i have to be activated, in order to get a focusing array of the seismic sources i of which the seismic waves arrive at the target point k in phase at time to.

[0001] The present invention relates to activating a seismic source suchthat the seismic energy received in a target point in an undergroundformation has a prescribed shape. A seismic source is a device thatreleases energy in the form of an acoustic wave. Basically there are twotypes of seismic sources, an impulsive seismic source of which thewaveform cannot be controlled and a waveform-controllable seismicsource. Examples of an impulsive seismic source are an explosive device,a dropping mass or an implosive device, and the signal generated by animpulsive seismic source is a pulse. The waveform of an impulsiveseismic source cannot be controlled. A waveform-controllable seismicsource is a vibrator that generates a controlled wave train. The seismicsources are used to make a seismic survey of the structure of anunderground formation.

[0002] Making a seismic survey of the structure of an undergroundformation comprises activating the seismic sources to generate seismicwaves; allowing the seismic waves to travel through the undergroundformation in which they are reflected by acoustic-impedance contrastsand refracted through members having a variable velocity; and receivingand recording the seismic waves by seismic receivers. The arrival timesof the seismic waves are observed to obtain a result that can beinterpreted.

[0003] It is an object of the present invention to use the informationobtained from the seismic survey to determine how to activate a seismicsource such that the seismic energy received in a target point in theunderground formation has a prescribed shape.

[0004] As observed above, there are two kinds of seismic sources,impulsive seismic sources and waveform-controllable seismic sources. Forthe impulsive seismic sources it is an object to concentrate the seismicenergy in a target point.

[0005] To this end the method of providing a set of seismic sources in afocusing array to enable concentrating seismic energy in a selectedtarget point in an underground formation according to the presentinvention comprises the steps of:

[0006] (a) performing a standard seismic survey of the undergroundformation using a set of impulsive seismic sources i and a set ofdownhole seismic receivers j, and recording the signals u_(ij)(t);

[0007] (b) selecting a position k to be the target point;

[0008] (c) determining from the signals u_(ij)(t) the one-way traveltimes τ_(ik) from a number of sources i to the target point at thelocation k; and

[0009] (d) calculating t_(i)=t₀−τ_(ik) to obtain the activating timest_(i) at which the impulsive seismic sources i have to be activated, inorder to get a focusing array of the seismic sources i of which theseismic waves arrive at the target point k in phase at time t₀.

[0010] This method is suitably applied in case the target pointcoincides with a seismic receiver k. However, in case the target point ldoes not coincide with a seismic receiver k, a correction has to beapplied. The correction on the activation time of the sources i isΔt_(i)=τ_(ik)−τ_(il), wherein τ_(il) is the one-way travel time fromsource i to the target point l. The corrected activation time for theseismic source i is then t^(corr) _(i)=t_(i)+Δt_(i).

[0011] Suitably the one-way travel times are calculated with a velocitymodel that is obtained from signals u_(im)(t), wherein m refers to a setof seismic receivers, at least one of which is located at surface.

[0012] For the waveform-controllable seismic sources it is an object ofthe present invention to enable concentrating seismic energy in aselected target point in an underground formation, wherein the wave atthe target point has a predetermined desired waveform.

[0013] To this end the method of providing at least one focusedwaveform-controllable seismic source to enable concentrating seismicenergy in a selected target point in an underground formation accordingto the present invention comprises the steps of:

[0014] (a) performing a standard seismic survey of the undergroundformation using the at least one waveform-controllable source i, and aset of downhole seismic receivers j, and recording the signalsu_(ij)(t), wherein each source is activated by a signal s_(i)(t);

[0015] (b) selecting a downhole seismic receiver k from the set ofseismic receivers to be the target point;

[0016] (c) taking the direct part u^(d) _(ik)(t) of the signal u_(ik)(t)and making an inverse filter (u^(d) _(ik)(t))⁻¹ of the direct part ofthe signal;

[0017] (d) selecting a desired waveform f_(k)(t−t₀) to be focused at thelocation of the downhole seismic receiver k at time t=t₀; and

[0018] (e) calculating the new signal with which the seismic source i isto be activated in accordance with s^(new) _(i)(t)=s_(i)(t)

(u^(d) _(ik)(t))⁻¹

f_(k)(t−t₀) to get a focused source i of which the waves arrive at thedownhole seismic receiver k with the desired waveform f_(k)(t) startingat time is to.

[0019] This method is applicable in case the target point coincides witha seismic receiver k. However, in case the target point does notcoincide with a seismic receiver k, a correction has to be applied. Tothis end the method of providing at least one focusedwaveform-controllable seismic source to enable concentrating seismicenergy in a selected target point in an underground formation accordingto the present invention comprises the steps of:

[0020] (a) performing a standard seismic survey of the undergroundformation using the at least one waveform-controllable source i, and aset of downhole seismic receivers j, and recording the signalsu_(ij)(t), wherein each source is activated by a signal s_(i)(t);

[0021] (b) selecting a position l to be the target point, wherein theposition l is near a downhole seismic receiver k of the set of seismicreceivers, and determining from the signals u_(ij)(t) the one-way traveltimes τ_(ik) from a number of sources i to the seismic receiver k nearthe target point at the location 1;

[0022] (c) taking the direct part u^(d) _(ik)(t) of the signal u_(ik)(t)and making an inverse filter (u^(d) _(ik)(t))⁻¹ of the direct part ofthe signal;

[0023] (d) selecting a desired waveform f_(k)(t−t₀) to be focused at thelocation of the downhole seismic receiver k at time t=t₀; and

[0024] (e) calculating the new signal with which the seismic source i isto be activated in accordance with s^(new) _(i)(t)=s_(i)(t)

(u^(d) _(ik)(t))⁻¹

f_(k)(t−t₀) to get a focused source i of which the waves arrive at thedownhole seismic receiver k with the desired wave form f_(k)(t) startingat time is t₀, and applying a correction on the signal with which theseismic source i is activated, wherein the correction is given byΔt^(corr) _(i)=τ_(ik)−τ_(il), τ_(il) being the estimated one-way traveltime from source i to the target point l, such that s^(newc)_(i)(t)=s^(new) _(i)(t+Δt_(i)).

[0025] Suitably, the one-way travel time to the target location iscalculated using a velocity model obtained from signals u_(im)(t),wherein m refers to a set of seismic receivers, at least one of which islocated at surface.

[0026] In the specification and the claims the symbol

denotes convolution.

[0027] The invention will now be described by way of example in moredetail with reference to the accompanying drawings, wherein

[0028]FIG. 1 shows an embodiment of the invention; and

[0029]FIG. 2 shows schematically how the activation times arecalculated.

[0030] Reference is now made to FIG. 1, showing schematically and not toscale a section 1 of the earth having a surface 2. At the surface 2 arelocated impulsive seismic sources 5, and in a borehole 6 are locateddownhole seismic receivers 8.

[0031] The first step according to the method according to the presentinvention is performing a standard seismic survey of the undergroundformation using a set of impulsive seismic sources i and a set ofdownhole seismic receivers j, and recording the signals u_(ij)(t). Thusfor each pair i,j there is one seismic signal u_(ij)(t). In theembodiment shown the set of impulsive seismic sources i consists of oneor more of the impulsive seismic sources 5. For the sake of clarity wehave indicated one of the sources with the reference i. The set ofdownhole seismic receivers j consists of one or more of the seismicreceivers 8, and for the sake of clarity we have indicated one of thereceivers with the reference j.

[0032] The next step is selecting a position that is to be used as atarget point. In this case we selected the dot k to represent theposition of the target point, which coincides with one of the receivers8.

[0033] The seismic signals u_(ij)(t) are used to make a velocity model,and that velocity model is used to calculate the one-way travel timesτ_(ik) from a number of sources i to the target point at the location k(which coincides with one of the seismic receivers 8). Please note thatmaking a velocity model and calculating the one-way travel times from itis well known, and will therefore not be discussed in thisspecification. The velocity model can also be obtained from signalsu_(im)(t), wherein m refers to a set of seismic receivers, at least oneof which is located at surface.

[0034] The one-way travel times τ_(ik) are now used to calculate theactivation times t_(i) at which the impulsive seismic sources i have tobe activated, in order to get a focusing array of the seismic sources iof which the seismic waves arrive at the target point k in phase at timet₀. The activation times t_(i) are calculated according to theexpression t_(i)=t₀−τ_(ik).

[0035]FIGS. 2a and 2 b show how the activation times for three impulsiveseismic sources arranged at the surface 2 of the formation 1 arecalculated. The wave paths from the sources to the receiver are referredto with reference numerals 10, 11 and 12. Reference numeral 15 refers toa fictitious trace that is a combination of the signals u₁₁, u₂₁ andu₃₁. When the impulsive seismic sources i=1, i=2 and i=3 are activatedat the same time, the signal recorded with seismic receiver k=1 showsthe arrivals 21, 22 and 23 of the three seismic waves. We do not showthe remainder of the signals consisting of reflections.

[0036] The one-way travel times are τ₁₁, τ₂₁ and τ₃₁, respectively,wherein (in this example) τ₂₁<τ₁₁<τ₃₁. To ensure that the seismic wavesarrive at the target point k in phase at time t₀—signal 25 in FIG.2b—the activation times must be calculated as shown in FIG. 2b, whereint₃<t₁<t₂.

[0037] Because the target point k coincides with a seismic receiver, theactivation times are so calculated that the seismic waves arrive at theseismic receiver at k in phase at time to. However if the target point kdoes not coincide with a seismic receiver the one-way travel timesτ_(ik) have to be estimated from the velocity model, or a correction hasto be applied.

[0038] Thus the invention gives a simple method for concentrating theseismic energy of a set of impulsive seismic sources in a target point.

[0039] An application of this method is creating a virtual source at theposition k, which may coincide with a seismic receiver 8. The wavesemitted from the virtual source at the position k can be used to get aseismic image of a formation layer 30, see FIG. 1. Moreover the signalfrom the receiver 8 at the position k will provide a check on thevalidity of the velocity model used to calculate the one-way traveltimes. Thus the method of the present invention offers the opportunityto check the validity of the velocity model, and it provides a methodfor providing an image of the formation layer 30 using a source k thatis closer to it then the sources 5.

[0040] Assume now that this formation layer 30 is an oil-bearing layerfrom which oil is produced via a well that is not shown in FIG. 1. Ifnow a target point l is selected inside the formation layer 30 theconcentrated seismic energy can be used to mobilize the oil in theformation layer 30 and thus the oil production is augmented.

[0041] The activation times used to ensure that the seismic waves arriveat the location l in phase at a time t₀ can be estimated from thevelocity model.

[0042] However, if we already know the activation times t_(i) that arecalculated to activate the impulsive seismic sources such that theseismic waves arrive at the seismic receiver at k in phase at time t₀,the activation times used to ensure that the seismic waves arrive at thelocation l in phase at time to can be found by a applying the followingcorrection Δt_(i)=τ_(ik)−τ_(il), wherein τ_(il) is the estimated one-waytravel time from source i to the target point l. Adding the correctionto the above equation for the activation times gives t^(corr)_(i)=t_(i)+Δt_(i)=t₀-τ_(ik)+τt_(i)=t₀−τ_(il), which is the activationtime that has to be used in order that the seismic waves arrive at thelocation l in phase at time t₀.

[0043] We will now discuss an embodiment of the present invention thatis applicable to the second group of seismic sources, thewaveform-controllable seismic sources. In this embodiment a mathematicalmethod is used to determine the signal with which the seismic source hasto be activated in order that a desired waveform f_(k)(t−t₀) is focusedat the location of the downhole seismic receiver k at time t=t₀. Themethod will be discussed with reference to FIG. 1, wherein a differencewith the above-described method is that the sources arewaveform-controllable seismic sources.

[0044] The first step of the method is as discussed above, performing astandard seismic survey of the underground formation using the at leastone waveform-controllable source i, and a set of downhole seismicreceivers j, and recording the signals u_(ij)(t), wherein each source isactivated by a signal s_(i)(t). Then a downhole seismic receiver k isselected from the set of seismic receivers to be the target point.

[0045] The next step is taking the direct part u^(d) _(ik)(t) of thesignal u_(ik)(t) and making an inverse filter (u^(d) _(ik)(t))⁻¹ of thedirect part of the signal.

[0046] Then the desired waveform f_(k)(t−t₀) has to be selected.

[0047] The new signal with which the seismic source i is to be activatedis calculated in accordance with the expression s^(new) _(i)(t)=s_(i)(t)

(u^(d) _(ik)(t))⁻¹

f_(k)(t−t₀). When the seismic source with this input signal, one obtainsa focused source i of which the waves arrive at the downhole seismicreceiver k with the desired waveform f_(k)(t) starting at time is t₀.

[0048] When more than one seismic source is used, the above describedmethod is applied to calculate for each seismic source i the new inputsignal s^(new) _(i)(t).

[0049] This method can be applied to create a virtual source at theposition k, which may coincide with a seismic receiver 8. The wavesemitted from the virtual source at the position k can be used to get aseismic image of a formation layer 30, see FIG. 1. Moreover the signalfrom the receiver 8 at the position k will provide a check on thevalidity of the velocity model used to calculate the one-way traveltimes. Thus the method of the present invention offers the opportunityto check the validity of the velocity model, and it provides a methodfor providing an image of the formation layer 30 using a source k thatis closer to it then the sources 5.

[0050] The step of taking the direct part of the signal and making aninverse filter of the direct part of the signal can be performed in thetime domain or in the frequency domain using Fourier Transforms.

[0051] The Fourier Transform of the direct part of the signal isU_(ik)^(d)(f) = ∫_(−∞)^(∞)u_(ik)^(d)(t) ⋅ ^(−2    π  f  t)t.

[0052] The inverse filter is (u^(d) _(ik)(t))⁻¹ so selected that itsFourier Transform is 1/U^(d) _(ik)(f).

[0053] For the sake of completeness, we would observe that ideally aspike is obtained at t=to when the inverse filter is convolved with thedelayed direct part of the signal. In equations: the convolution is(u^(d) _(ik)(t))⁻¹

(u^(d) _(ik)(t−t₀)), the Fourier Transform of the delayed direct partu^(d) _(ik)(t−t₀) of the signal is (e^(i2πft) ₀). (U^(d) _(ik)(f)), andthe Fourier Transform of the convolution is e^(−i2πft) ₀. The inverseFourier transform of e^(−2πft) ₀ is δ(t−t₀), which is the Dirac deltafunction at t=t₀.

[0054] The desired waveform at t=t₀ can be obtained by convolving theDirac delta function at t=t₀ with the desired waveform, in equation:

f _(k)(t−t ₀)=δ(t−t ₀)

f _(k)(t)

[0055] Given the limited bandwidth of the data, the inverse filter,(u^(d) _(ik)(t))⁻¹, is suitably stabilized by convolving it with a bandlimited desired response.

[0056] The above-described method is applicable in case the target pointcoincides with a seismic receiver k. However, in case the target pointdoes not coincide with a seismic receiver k, a correction has to beapplied. And this embodiment of the invention will now be discussed.

[0057] The first step of the method wherein the correction is applied isperforming a standard seismic survey of the underground formation usingthe at least one waveform-controllable source i, and a set of downholeseismic receivers j, and recording the signals u_(ij)(t), wherein eachsource is activated by a signal s_(i)(t). Then a position l is selectedto be the target point, wherein the position l is near a downholeseismic receiver k of the set of seismic receivers, and determining theone-way travel times τ_(ik) from a number of sources i to the seismicreceiver k near the target point at the location l. Subsequently thedirect part u^(d) _(ik)(t) of the signal u_(ik)(t) is used to make aninverse filter (u^(d) _(ik)(t))⁻¹ of it.

[0058] A desired waveform f_(k)(t−t₀) to be focused at the location ofthe downhole seismic receiver k at time t=t₀ is selected. The new signalwith which the seismic source i is to be activated is calculated inaccordance with the expression s^(new) _(i)(t)=s_(i)(t)

(u^(d) _(ik)(t))⁻¹

f_(k)(t−t₀) The result is a focused source i of which the waves arriveat the downhole seismic receiver k with the desired waveform f_(k)(t)starting at time is to. However, because the target point l does notcoincide with the seismic receiver k, a correction on the signal has tobe applied. The correction on the signal of the source(s) i is a timeshift Δt^(corr) _(i)=τ_(ik)−τ_(il), wherein τ_(il) is the estimatedone-way travel time from source i to the target point l. The correctednew signal becomes s^(newc) _(i)(t)=s^(new) _(i)(t+Δt^(corr) _(i))

[0059] Suitably, determining the one-way travel time to the target pointl comprises obtaining from the signals u_(ij)(t) a velocity model, andcalculating the one-way travel times τ_(il) with the velocity model.

[0060] This method can for example be applied to concentrate seismicenergy in a target point l in an oil-bearing formation layer 30 in orderto mobilize the oil in the formation layer 30.

[0061] The focused set of seismic sources is suitably used to providehigh energy at the target point. Applications may include enhancing oilproduction by increased mobilization in area around the target point;enhancing the signal-to-noise ratio of seismic imaging; enhancing thesignal to noise ratio of seismic-electric effects in which the seismicsignals are used to generate detectable electromagnetic responses; andtransmission of signals or energy to remote devices.

1. A method of providing a set of seismic sources in a focusing array to enable concentrating seismic energy in a selected target point in an underground formation, which method comprises the steps of: (a) performing a standard seismic survey of the underground formation using a set of impulsive seismic sources i and a set of downhole seismic receivers j, and recording the signals u_(ij)(t); (b) selecting a position k to be the target point; (c) determining from the signals u_(ij)(t) the one-way travel times τ_(ik) from a number of sources i to the target point at the location k; and (d) calculating t_(i)=t₀−τ_(ik) to obtain the activating times t_(i) at which the impulsive seismic sources i have to be activated, in order to get a focusing array of the seismic sources i of which the seismic waves arrive at the target point k in phase at time t₀.
 2. The method according to claim 1, wherein a correction Δt_(i)=τ_(ik)−τ_(il) on the activation time of the sources i is applied for a target point l differing from the target point k, wherein τ_(il) is the one-way travel time from source i to the target point l.
 3. The method according to claim 1, wherein determining the one-way travel time to the target point comprises obtaining from signals u_(im)(t) a velocity model, and calculating the one-way travel times from the seismic sources i to the target point with the velocity model, wherein m refers to a set of seismic receivers, at least one of which is located at surface.
 4. The method according to claim 2, wherein determining the one-way travel time to the target point comprises obtaining from signals u_(im)(t) a velocity model, and calculating the one-way travel times from the seismic sources i to the target point with the velocity model, wherein m refers to a set of seismic receivers, at least one of which is located at surface.
 5. A method of providing at least one focused waveform-controllable seismic source to enable concentrating seismic energy in a selected target point in an underground formation, which method comprises the steps of: (a) performing a standard seismic survey of the underground formation using the at least one waveform-controllable source i, and a set of downhole seismic receivers j, and recording the signals u_(ij)(t), wherein each source is activated by a signal s_(i)(t); (b) selecting a downhole seismic receiver k from the set of seismic receivers to be the target point; (c) taking the direct part u^(d) _(ik)(t) of the signal u_(ik)(t) and making an inverse filter (u^(d) _(ik)(t))⁻¹ of the direct part of the signal; (d) selecting a desired waveform f_(k)(t−t₀) to be focused at the location of the downhole seismic receiver k at time t=t₀; and (e) calculating the new signal with which the seismic source i is to be activated in accordance with s^(new) _(i)(t)=^(s) _(i)(t)

(u^(d) _(ik)(t))⁻¹

f_(k)(t−t₀) to get a focused source i of which the waves arrive at the downhole seismic receiver k with the desired waveform f_(k)(t) starting at time is to.
 6. A method of providing at least one focused waveform-controllable seismic source to enable concentrating seismic energy in a selected target point in an underground formation, which method comprises the steps of: (a) performing a standard seismic survey of the underground formation using the at least one waveform-controllable source i, and a set of downhole seismic receivers j, and recording the signals u_(ij)(t), wherein each source is activated by a signal s_(i)(t); (b) selecting a position l to be the target point, wherein the position l is near a downhole seismic receiver k of the set of seismic receivers, and determining from the signals u_(ij)(t) the one-way travel times τ_(ik) from a number of sources i to the seismic receiver k near the target point at the location l; (c) taking the direct part u^(d) _(ik)(t) of the signal u_(ik)(t) and making an inverse filter (u^(d) _(ik)(t))⁻¹ of the direct part of the signal; (d) selecting a desired waveform f_(k)(t−t₀) to be focused at the location of the downhole seismic receiver k at time t=t₀; and (e) calculating the new signal with which the seismic source i is to be activated in accordance with s^(new) _(i)(t)=s_(i)(t)

(u^(d) _(ik)(t))⁻¹

f_(k)(t−t₀) to get a focused source i of which the waves arrive at the downhole seismic receiver k with the desired wave form f_(k)(t) starting at time is to, and applying a correction on the signal with which the seismic source i is activated, wherein the correction is given by Δt^(corr) _(i)=τ_(ik)−Σ_(il), τ_(il) being the estimated one-way travel time from source i to the target point l, such that s^(newc) _(i)(t)=s^(new) _(i)(t−Δ^(corr)t_(i)).
 7. The method according to claim 6, wherein determining the one-way travel time to the target point l comprises obtaining from signals u_(im)(t) a velocity model, and calculating the one-way travel times τ_(il) with the velocity model, wherein m refers to a set of seismic receivers, at least one of which is located at surface. 